Augustica
®
T e c h n o l o g i e s
Page 3 of 7
I
NTRODUCTION
Tube circuit experts know that even power tubes that can deliver several hundred
milliamperes of anode current can never drive a loudspeaker with an impedance of 8 Ω,
since the internal impedance of a tube is several kiloohms. The two impedances are thus
almost always matched using a transformer. In principle, there is no any objection to
using this form of impedance matching, but every transformer tends to degrade the sound
quality. Keeping this effect to a minimum requires very careful and complicated coil
winding techniques, good-quality transformer iron and large core cross-sections.
However, if the load impedance is not just a few ohms but instead several hundred ohms,
as is the case with many types of headphones, and if in addition the required power level
is not overly high, an amplifier with no output transformer - sometimes referred to as an
‘output transformerless’ (OTL) amplifier - can be a feasible option. In such a case, the
load is driven directly by the tubes.
The headphone amplifier kit Corvette
®
allows building a headphone amplifier with
outstanding sound. Corvette
®
omits the output transformer, avoids overall negative
feedback and uses only high quality coupling capacitors, resistors and other components.
The Electro Harmonix
®
6922 tubes in left and right channels are closely matched,
therefore, the amplifier guarantees a highly linear frequency characteristic and low
crosstalk between the left and the right channels. The headphone amplifier Corvette
®
employs a cathode follower design suitable for use with a single or dual headphone, with
each headphone element having impedance between 15 and 300 Ω for the total of 30 to
600 Ω.
A
MPLIFIER
- T
HEORY OF
O
PERATION
The circuit of the headphone amplifier Corvette
®
is shown in
Figure 1
(Corvette's
schematic also appears on the website www.Augustica.com) uses the readily available
Electro Harmonix
®
6922 double triode to provide amplification. For the purposes of this
discussion, we will be considering left channel of the amplifier. The right channel of the
amplifier Corvette
®
is identical to the left channel. A preamplifier stage is necessary to
generate signal amplitudes sufficient to drive a headphone. The first triode section with
base pins 1, 2 and 3 is used for this purpose. The input signal arrives to the circuit board
via a 100-kilo Ω logarithmic potentiometer R1 that serves as a volume control and is
directly coupled to the preamplifier stage via capacitor C1. Resistor R2 provides the
necessary negative grid bias. Resistor R5 determines the gain, while resistor R4
determines the maximum input voltage. The value of the resistor R5 is selected to assure
that the quiescent anode current is situated in the most linear possible portion of the
characteristic curve for 6922.
The inverted and amplified input signal on the anode of the first section of 6922 is
coupled to the grid of the second section via capacitor C2. The cathode resistor of the
second section is split into two resistors R7 and R8. The series resistance of R7 and R8
